When measuring the relative moisture content, cells of different types are used. For example, cells are used for measuring the relative moisture content in building construction, when it is necessary to determine when in situ concrete is sufficiently dry to be able to put a covering on the concrete, for example, without risk of moisture and/or mould damage. When using known cells for measuring the relative moisture content, problems have arisen with the accuracy of the cells. Known cells can undergo changes after calibration without this being noticed, or they can be inadvertently read before a vapour balance between the cell and the concrete has been reached. In some cases, the accuracy is so poor that use is limited to sensing free water in the vicinity of the cell. This is for example the case when using the device described in Swedish Application B-375395.
When measuring the relative moisture content of concrete, it is desirable that measuring error does not exceed one percentage unit within the measuring range 85-98% relative moisture. This cannot be achieved with previously known cells, because the accuracy of these cells is too low to achieve this. Furthermore, significant measuring errors can be caused by the fact that insertion of sensors with known cells in the concrete changes the temperature in the concrete.
The purpose of the present invention is to provide a conductivity or capacitivity cell of the type described by way of introduction which fulfills the accuracy requirement specified above, and a method of producing such a cell, which makes it possible to manufacture simply and inexpensively a large number of cells with very little variation as regards properties.
Another purpose of the invention is to provide a sensor for measuring the relative moisture content of a material, preferably concrete, said sensor using a cell according to the invention to achieve the desired accuracy, and a method for measuring the relative moisture content of a material to a desired accuracy.